The present invention relates to a system and method for starting a synchronous motor from a static inverter without reliance upon rotor position and speed measurements. The proposed system provides a method for performing a start of an auxiliary power unit (APU) for jet aircraft. Prior art includes the use of a DC motor to start the APU from a battery.
Exemplary of the prior art is U.S. Pat. No. 4,361,791 (Allan B. Plunkett) which describes an apparatus for controlling a permanent magnet synchronous motor driven by a pulse width modulated inverter. The method forms a modified flux vector by phase shifting the measured flux vector, and using this modified flux vector as a feedback signal for inverter control. Further exemplary of the prior art is U.S. Pat. No. 4,855,519 (Heinrich-Karl Vogelmann) which describes an apparatus for determining the flux vector of a machine without using a mechanical shaft position indicator. This is done by injecting a component of stator current from which the error between the computed flux vector and the actual flux vector can be calculated. The error is used to modify the computed flux vector. In contrast, the proposed system utilizes a flux estimation system using well known synchronous machine equations with feedback added to ensure stability of the estimated flux.
The proposed system utilizes a pulse-width-modulated static inverter to produce a controlled AC power to drive the generator of the APU as a synchronous motor for starting. An important feature of the present invention includes the method in which the output of the inverter is modified as the APU is started using only electrical system measurements. Rather than measuring the rotor speed and position directly in an attempt to optimize the starting torque, the supply current to the stator is measured, filtered and is used as a feedback to estimate the motor magnetizing flux. Then, by adjusting the supply voltage to hold the phase angle between the current and the estimated flux at the proper angle, the starting current can be optimized. An additional restriction is applied to avoid exceeding the maximum supply voltage.